Well, in one episode, Amy supposedly tricked them into staying stone by walking around and acting like she could see.
Also, they are "quantum locked", which suggests that they are really immune to any form of observation, including hearing.

It's a good analogy. An interpretation like that would suggest that they are allowed to move as long as there is no way to determine the exact path they took. However, if this were taken in the most literal sense, they would almost always be frozen -- so there are some limitations to make the show work.

I see. Perhaps their concept of perception is "If they can see me." thats what I would think. I bet the angels could even turn that off, with the proper thought process! ...im bored and rambling. bye. :3